You can charge whatever you want. The other person may or not be willing to pay, but you don't have to accept the job if you aren't happy with the price. It's best if you can estimate how long the piecing will take you, so you have a good idea of how big a chunk of your life it will take up. It could be that you are happy to do it and accept very little per hour, or that you really don't want to do it unless you get a LOT per hour. Either way, it's your decision.